The present invention relates to an apparatus for performing a desired logical function.
A gate array and a PLA (Programmable Logic Array) are well known as a device for performing a desired logical function. The gate array is a device which becomes a logic circuit performing a desired logic function by changing a metalization pattern formed on a master substrate having unit elements. The PLA is a logic array which is programmable to provide a desired logic function. There are two types of the PLA a mask PLA and an FPLA (Field-PLA). The mask PLA is a PLA which is programmed by a manufacurer. The FPLA is a PLA which can be programmed by a user.
In order to produce the gate array and mask PLA of a different logic function, it is necessary to use at least one different mask pattern whereas most of mask patterns are common. Although the gate array and mask PLA are more suitable for small scale production than a custom order IC, they are not suitable for very small scale production, such as a few devices. This is because producing a new mask pattern is costly.
The FPLAs are differently programmable from each other by a user to perform a different logical function. Programming may be performed by passing large enough current through fuses at intersections to melt them, or by passing large enough current through diodes at intersections to destroy them. As mentioned above, programming requires flowing large current which produces much heat in the FPLA. This results in the problem that it is difficult to improve integration. Another problem is that the FPLA after programming can be used for only one purpose. This is because a logical function generated by programming cannot be changed to another logical function.